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Current sensing illustrates engineering tradeoff

Sensing voltage is usually straightforward, at least in terms of physical access; you just connect to the two points representing the potential difference you want to measure. Depending on the situation, you may need a floating or isolated interface, but at least you have basic access, without the need for any additional in-circuit sensor components.

Current, on the other hand, can be trickier to sense. You either have to break the supply line and insert a series resistor, and then measure the voltage across it differentially, or use a loop transformer or Hall-effect device as less-invasive pickoff.

Each approach has pros and cons. For modest currents, the sense resistor is low-cost and physically small, but picking the right resistance value requires balancing sensitivity, range and, perhaps most painful of all, wasted power (via I2R loss) and voltage drop (via IR loss). A smaller-value resistor minimizes both losses, but also yields a much smaller voltage output, which in turn mandates a better amplifier and also makes low-end readings more susceptible to noise. A larger-value resistor greatly eases the sensing and sensitivity problems, but can really hurt on the circuit efficiency and supply-rail side of the story. Finding that best value for the sensing resistor means weighing the tradeoffs versus your priorities.

The current-loop or Hall pickup has its own issues. It's sometimes harder to physically implement on a PC board, and a pickup loop itself is usually larger than a series resistor in the same situation. It needs a properly matched current-to-voltage amplifier, although sensing small current values is less noise-prone than small voltages. But it's inherently isolated, which simplifies circuit-level issues and concerns in some designs, and is mandatory in others. And of course, it inflicts no IR drop.

As in most engineering decisions, there's no right answer, it depends on the priorities in your application and the pain-versus-figurative gain balance of your project. This current-sense situation is a small and well-constrained example of the dozens of design tradeoffs and decisions engineers must make. It's too bad that those folks who so casually say “what's the big deal to do/add/change XYZ in the product?” won't understand, even from this modest example.

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